This invention is in the field of protective devices for photo-detectors, and is particularly concerned with protecting such detectors against laser countermeasures. The types of detectors being protected include image intensifiers, television camera tubes, infrared detectors, and the human eye. Because of its low damage threshold, the human eye is the most difficult to protect. Various techniques have been used and suggested as protection against laser radiation and other high-intensity radiations, such as searchlights and sunlight. The main techniques so used and suggested include various mechanical and electro-optical shutters, and for narrow-band radiation such as from lasers, absorbing or reflecting interference filters. A recent invention by one of the instant inventors (Shurtz) shows two embodiments of a frustrated total internal reflection limiter wherein high optical power induces thermal effects and is totally reflected away from a detector. This invention bears Ser. No. 492,062 and was filed Apr. 28, 1983.
Unfortunately, these techniques have disadvantages which make them unusable for some detectors and against some high-intensity sources. Specifically, shutters require relatively long times to open and close, compared to the rise time of a pulsed laser. Interference filters, in order to block the spectra of various lasers, must be individually made of each expected threatening laser, and must be stacked in the optical path of the detector. For low-light detection, such stacks cause acceptable attenuation. The Shurtz invention mentioned above is fast acting compared to shutters, and is wide band, but is slow compared to the instant invention.